On matters of addressing people of superior rank who hold several different ranks and titles ...?

... suppose you had a man who was an archduke of an empire, a colonel in the imperial army, and director of imperial intelligence. He has an adjutant, perhaps ranked captain in the army, who is reporting to him on an intelligence matter. Would the adjutant address him as a) a captain to a colonel, b) an intelligence agent to his director, or c) by his title as the archduke, second only in noble rank to literal sons of the emperor? What would the protocol be, and what form of address would he use?